You need a flashlight and a dark room for this project. Now take your kit to a dark room along with a flashlight. Turn power ON and shine the flashlight directly at the CdS cell. Now move your hand in front of the flashlight beam and see what happens to the LED.
Try adjusting the control volume for different results. This is an example of a motion detector. These are often used in banks, defense plants, military bases and other areas where only certain people are admitted. This lets us detect intruders without the intruders being aware of it.
The beam of light is usually located so that the intruder can't easily see it (for example, it can be only a few inches off the floor). Sometimes we use light invisible to the human eye, such as infrared, to make sure the intruder doesn't know about the motion detector. Try using this detector in a normal room. Does it work?